In recent years, mobile wireless communications have become increasingly popular. Initial implementations of mobile wireless communications, for example in the form of cellular telephone networks, supported circuit switched voice communication services. Today wireless carriers also offer packet data communication services to their mobile customers. Today's cellular network architecture is evolving from a circuit-switched, voice-centric architecture towards an IP-based architecture supporting voice, video, and data services. Several new technologies have evolved to support the future architecture.
Networks offering circuit switched voice communication services utilize mobile telephone switching equipment specially adapted to handle communications through base stations for over-the-air communications with mobile telephones. The mobile switching center (MSC), for example, provides circuit-switched links to the public switched telephone network (PSTN) and provides circuit switching of ongoing calls between base stations in support of handoff between base stations. For service control, a public mobile wireless network includes a Home Location Register (HLR), which stores data regarding the valid station's identification, the assigned telephone number, subscription service options terminal capabilities, etc. for each mobile station homed to the network. The home network uses the service information from the HLR to provide the subscribed services to each user's mobile station while the station is operating in the service area of the home network. To facilitate roaming and attendant registration for voice services, each network also implements a Visitor Location Register (VLR). A VLR is a location register, which an MSC temporarily uses to store and retrieve information regarding a visiting user's mobile station. The VLR and the HLR interact to validate each roaming mobile station. For a validated station, service information for that station is downloaded from the HLR to the VLR in a visited access network during a successful registration process. The validation process also provides information to the HLR indicating the current location of the station, to allow the home network to route incoming voice calls to the station at its current location.
Packet-based services have been offered via an overlay on such a circuit-switching voice-centric wireless network architecture as well as a data optimized RAN called the Packet Data Subsystem (PDS). In a typical example, a receiving node, such as a packet data serving node (PDSN), handles packet data sessions. In an initial implementation, the MSC provides a link to the PDSN, which serves as the edge of the IP packet domain. Upon receiving a packet data call, the PDSN accesses an authentication, authorization and accounting (AAA) server to obtain call access authorization. The PDSN can operate in two modes: Simple IP and Mobile IP. In Simple IP implementations, the PDSN assigns the IP address. In Mobile IP network implementations, mobile IP (MIP) address service enables routing of packets between PDSNs, to effectively enable roaming between service areas of different PDSNs. At log-in, a home agent (HA) assigns an address to the station, from the home carrier's pool of addresses, for use during the duration of the session. When a mobile station has roamed across a PDSN boundary, the mobile station obtains packet data services via a different PDSN, and the mobile station obtains a “care-of-address” (COA) from a local Foreign Agent (FA) in the visited region. The registration/validation process provides notice of this COA to the station's Home Agent (HA) in its home network. Although other control nodes or routers may perform these Agent functions, often they are implemented in the PDSNs. The COA address allows the PDSN-HA to route incoming packets for the roaming mobile station arriving with the assigned mobile address through the PDSN-FA router and the visited network, and hence, to the roaming mobile station.
As an upgrade or migration from the above architecture, the 3GPP standards body developed a framework for an IP Multimedia Subsystem (IMS) that would be a services overlay on the existing radio access network (RAN) architecture but would provide end-to-end IP transport, in most cases, without circuit switching for voice traffic. 3GPP does not use Mobile IP and does not have PDSN and HA functions, but it provides analogous mechanisms through SGSN and GGSN. The focus of the IMS development was on voice services over IP, and operator inter-working for voice services is extensively addressed. 3GPP was also a framework to quickly adapt solutions from multiple vendors under the same umbrella.
3GPP2 adopted the IMS framework from 3GPP, and in conjunction with the PDS is referred to as the ‘Multi-Media Domain’ (MMD). Several IMS paradigms are different from the ANSI-41 circuit switched model utilized today for much of the mobile voice traffic and similar in some aspects to the PDS paradigms. For example:                The IMS architecture separates control, bearer and database functions as compared to the ANSI-41 architecture where the control, bearer and database were provided in the same network element (Integrated MSC, VLR, HLR). This separation needs new network design and operational guidelines/principles.        IMS is based on ‘home network control’ similar to the PDS as opposed to the ‘visited network control’ paradigm of ANSI-41 for all real-time as well as data services. This may have significant impact on call/session setup times and the bearer latency based on the geographic distances between the home and visited networks.        IMS is based on ‘device intelligence’ similar to the Internet paradigm as opposed to ‘network intelligence’ in ANSI-41. This ignores the conditions of the end-to-end network which provides services to millions of subscribers and which will impact user experience.        
FIG. 1 shows the functional specification as is currently defined in the 3GPP2 standard documentation. The top part of the diagram shows the functions of IMS and bottom part shows the functions of PDS. Some functions are common to both subsystems.
The different functions as defined in the existing standards are described below, although some of the functions are not defined yet in the standards and interpretation varies in the industry.
There are a number of Call Session Control Functions (CSCFs). The Proxy Call Session Control Function (P-CSCF) enables the session control to be passed to a Serving CSCF. The Serving CSCF (S-CSCF) is in the home network and invokes the service logic. The Interrogating CSCF (I-CSCF) identifies the S-CSCF associated with the subscriber and also identifies the terminating S-CSCF in a visited network. However, the Serving CSCF (S-CSCF) does not handle service interaction issues as is defined today.
The Home Subscriber Server (HSS) is a home AAA entity and associated databases for IMS related services.
A Media Gateway (MGW) provides an interface between a TDM network, for example, of the PSTN, and the IP network. The Media Gateway Controller Function (MGCF) controls the media gateway. The Breakout Gateway Control Function (BGCF) selects which MGW is to be used for a communication to/from the PSTN as well as other mobile networks. The Signaling Gateway (SGW) provides an interface between SS7 and IP-based signaling.
The Media Resource Function Processor (MRFP) provides media resources like announcements, media streaming, conferencing, etc., in association with or in support of multimedia services. The Media Resource Function Controller (MRFC) controls the MRFP.
The Authentication, Authorization, Accounting (AAA), Home Agent (HA) and Foreign Agent (FA) would function essentially as outlined above.
The Access Gateway (AGW) is the PDSN/FA described above.
The Interconnectivity Core Network-Bearer Control Point (ICN-BCP) is an entity through which IP-connectivity Network flows pass. The ICN-BCP is able to control the allocation of IP-connectivity network bearer resources to the AT and receives QoS and bandwidth related requests from IMS services being invoked by the subscriber.
The Policy Decision Function (PDF) defined today is based on user policies for data services. The functionality for IMS related services is not yet standardized and has wide interpretation in the industry for personalization of IMS services on a user basis.
The Position Determining Entity (PDE) manages geographic location determinations for access terminals, particularly for mobile access terminals. The Position Server in turn provides access to determined geographical terminal location information. These are not yet defined in detail in the Standards.
The standard also identifies the following function elements but has not yet clearly defined specific functions:                The SIP Application Server (SIP AS)        The Open Services Access Service Capability Server (OSA SCS)        The Open Services Access Application Server (OSA AS)        The Services Capability Interaction Manager (SCIM)        
Key features of the 3GPP2/MMD architecture, one or more of which may raise concerns of importance to network operators, include the following:                The call/session is distributed among several execution environment servers without any operator management capabilities that will be required in order to manage the network features like bandwidth and QoS        Call/session control is in the device, which will not allow the operator to manage user experience during changing conditions of the network while the user is mobile        The HSS includes subscriber databases and some service related functions        Access (read/write) of data into the HSS is provided to all internal and external Application Servers—which may not be desirable from an Operator's perspective in order to maintain security        Absence of Services/feature interaction management capability to add/drop/modify services as deemed appropriate based on user profile and network conditions        There is no uniform data management of permanent user data, and transient data needed for the current session execution        The network operator will need to support subscriber to login from multiple devices (phone, PDA, Laptop) simultaneously and to support simultaneous voice and data from each. The filter criteria required to access profile information is not adequately defined in Standards.        
Current converged multimedia services delivery platforms based on the IP Multimedia Subsystem (IMS) have several shortcomings. First and foremost, IMS assumes all future services will be based on Session Initiation Protocol (SIP) whereas in reality SIP and non-SIP services like video/music content and games will co-exist. Therefore IMS cannot control non-SIP services and manage run-time interactions between simultaneous SIP and non-SIP services.
A subscriber may have more than one AT device, and the AT devices may take different forms, e.g. mobile handset station, laptop, portable digital assistant, etc. Each subscriber will obtain a variety of different services through the network. The network operator not only manages the network but also offers some of the services that are available through the network. To provide the user experience that customers expect, the network operator needs to track the state of every service and delivery thereof to all of the user's devices. IMS provides SIP call/session control distributed among several execution environment servers and the user device, which limits the Service Provider in managing bandwidth, QoS, and experience of the mobile user. Furthermore, IMS does not address network policy controls, which are important in managing services delivery across millions of users.
Also, in current networks, various security mechanisms (like user security, application security, network security) are not addressed in a comprehensive way. For example, authentication and encryption at access level, network level, and application level has significant impact on user experience. Also network admission control is not addressed.
In current Standards, mobility management is defined at the link (L2) layer, the network/IP (L3) layer and the application layer. The excessive number of network elements for mobility management is not operationally desirable.
The current standards-based architecture assumes AT device intelligence, whereas a network operator providing services through the network needs a greater degree of network intelligence. The current standards-based architecture assumes home network control, whereas the service provider will likely need visited network control for services like Emergency Services. Also, from the network operator's perspective, there are considerable advantages to centralizing as many functions as possible, which is contrary to the paradigm of the current standards-based architecture.